In the fabrication of light-emitting diodes (LEDs) and other high-performance devices such as laser diodes, optical detectors, and field effect transistors, a chemical vapor deposition (CVD) process is typically used to grow a thin film stack structure using materials such as gallium nitride over a sapphire or silicon substrate. A CVD tool includes a process chamber, which is a sealed environment that allows infused gases to be deposited upon the substrate (typically in the form of wafers) to grow the thin film layers. An example of a current product line of such manufacturing equipment is the TurboDisc® family of metal organic chemical vapor deposition (MOCVD) systems, manufactured by Veeco Instruments Inc. of Plainview, N.Y.
A number of process parameters are controlled, such as temperature, pressure, and gas flow rate, to achieve a desired crystal growth. Different layers are grown using varying materials and process parameters. For example, devices formed from compound semiconductors such as III-V semiconductors are typically formed by growing successive layers of the compound semiconductor using MOCVD. In this process, the wafers are exposed to a combination of gases, including a metal organic compound as a source of a group III metal, and also including a source of a group V element which flow over the surface of the wafer while the wafer is maintained at an elevated temperature. Generally, the metal organic compound and group V source are combined with a carrier gas, which does not participate appreciably in the reaction as, for example, nitrogen. One example of a III-V semiconductor is gallium nitride, which can be formed by reaction of an organo-gallium compound and ammonia on a substrate having a suitable crystal lattice spacing, as for example, a sapphire wafer. The wafer is usually maintained at a temperature on the order of 1000-1100° C. during deposition of gallium nitride and related compounds.
In MOCVD processing, where the growth of crystals occurs by chemical reaction on the surface of the substrate, the process parameters must be tightly controlled to ensure that the chemical reaction proceeds under the required conditions. Even small variations in process conditions can adversely affect device quality and production yield. For instance, if a gallium and indium nitride layer is deposited, variations in wafer surface temperature will cause variations in the composition and bandgap of the deposited layer. Because indium has a relatively high vapor pressure, the deposited layer will have a lower proportion of indium and a greater bandgap in those regions of the wafer where the surface temperature is higher. If the deposited layer is an active, light-emitting layer of an LED structure, the emission wavelength of the LEDs formed from the wafer will also vary to an unacceptable degree.
In an MOCVD processing chamber, semiconductor wafers on which layers of thin film are to be grown are placed on rapidly-rotating carousels, referred to as wafer carriers, to provide a uniform exposure of their surfaces to the atmosphere within the reactor chamber for the deposition of the semiconductor materials. Rotation speed is on the order of 1,000 RPM. The wafer carriers are typically machined out of a highly thermally conductive material such as graphite, and are often coated with a protective layer of a material such as silicon carbide. Each wafer carrier has a set of circular indentations, or pockets, in its top surface in which individual wafers are placed. Typically, the wafers are supported in spaced relationship to the bottom surface of each of the pockets to permit the flow of gas around the edges of the wafer. Some examples of pertinent technology are described in U.S. Patent Application Publication No. 2012/0040097, U.S. Pat. No. 8,092,599, U.S. Pat. No. 8,021,487, U.S. Patent Application Publication No. 2007/0186853, U.S. Pat. No. 6,902,623, U.S. Pat. No. 6,506,252, and U.S. Pat. No. 6,492,625, the disclosures of which are incorporated by reference herein.
The wafer carrier is supported on a spindle within the reaction chamber so that the top surface of the wafer carrier having the exposed surfaces of the wafers faces upwardly toward a gas distribution device. While the spindle is rotated, the gas is directed downwardly onto the top surface of the wafer carrier and flows across the top surface toward the periphery of the wafer carrier. The used gas is evacuated from the reaction chamber through ports disposed below the wafer carrier. The wafer carrier is maintained at the desired elevated temperature by heating elements, typically electrical resistive heating elements disposed below the bottom surface of the wafer carrier. These heating elements are maintained at a temperature above the desired temperature of the wafer surfaces, whereas the gas distribution device typically is maintained at a temperature well below the desired reaction temperature so as to prevent premature reaction of the gases. Therefore, heat is transferred from the heating elements to the bottom surface of the wafer carrier and flows upwardly through the wafer carrier to the individual wafers. The gas flow over the wafers varies depending on the radial position of each wafer, with outermost-positioned wafers being subjected to higher flow rates due to their faster velocity during rotation. Even each individual wafer can have temperature non-uniformities, i.e., cold spots and hot spots depending upon its geometrical position relative to the other wafers on the carrier.
During MOCVD processing, the wafer carrier is predominantly heated by radiation, with the radiant energy impinging on the bottom of the carrier. For example, a cold-wall CVD reactor design (i.e., one that uses non-isothermal heating from the bottom) creates conditions in the reaction chamber where a top surface of the wafer carrier is cooler than the bottom surface. The degree of radiative emission from the wafer carrier is determined by the emissivity of the carrier and the surrounding components. Changing the interior components of the reaction chamber such as the cold-plate, confined inlet flange, shutter, and other regions, to a higher emissivity material can result in increased radiative heat transfer. Likewise, reducing the emissivity of the carrier will result in less radiative heat removal from the carrier. The degree of convective cooling of the carrier surface is driven by the overall gas flow pumping through the chamber, along with the heat capacity of the gas mixture (H2, N2, NH3, OMs, etc.). Additionally, introducing a wafer, such as a sapphire wafer, in a pocket can enhance the transverse component of the thermal streamlines, resulting in a “blanketing” effect. This phenomenon results in a radial thermal profile at the pocket floor that is hotter in the center and lower towards the outer radius of the pocket.
This non-uniform temperature profile on the surface of the wafer, which is compounded by centripetal forces during rotation (i.e., the “proximity” effect), can significantly decrease semiconductor production yield. Thus, a great deal of effort has been devoted to designing a system with features to minimize temperature variations during processing. Given the extreme conditions wafers are subject to during MOCVD processing, and the impact these conditions have on production yield, there remains a need for improved technologies to further reduce temperature non-uniformities.